Identifying and understanding the risk factors associated with diabetes is invaluable for the development and evaluation of effective intervention strategies. Lacking normal regulatory mechanisms, diabetics are encouraged to strive for optimal control through a modulated life style approach that focuses on dietary control, exercise, and glucose self-testing with the timely administration of insulin or oral hypoglycemic medications. Invasive forms of self-testing are painful and fraught with a multitude of psychosocial hurdles, and are resisted by most diabetics. Alternatives to the currently available invasive blood glucose testing are highly desirable.
Conventional approaches seek to reduce or eliminate the skin trauma, pain, and blood waste associated with traditional invasive glucose monitoring technologies. In general, noninvasive optical blood glucose monitoring requires no samples and involves external irradiation with electromagnetic radiation and measurement of the resulting optical flux. Glucose levels are derived from the spectral information following comparison to reference spectra for glucose and background interferants, reference calibrants, and/or application of advanced signal processing mathematical algorithms. Candidate radiation-based technologies include: 1) mid-infrared (MIR) spectroscopy, 2) near-infrared (NIR) spectroscopy, 3) far-infrared (FIR) spectroscopy, 4) radio wave impedance, 5) infrared photoacoustic spectroscopy and 6) Raman spectroscopy. Each of these methods uses optical sensors, and relies on the premise that the absorption pattern of infrared light (700-3000 nm) can be quantitatively related to the glucose concentration. Other substances such as water, protein, and hemoglobin are known to absorb infrared light at these wavelengths and easily obscure the relatively weak glucose signal.
Other approaches are based on microvascular changes in the retina; acoustical impedance, NMR spectroscopy and optical hydrogels that quantify glucose levels in tear fluid. While putatively noninvasive, these technologies have yet to be demonstrated as viable in clinical testing.
Nearly noninvasive techniques tend to rely on interstitial fluid extraction from skin. This can be accomplished using permeability enhancers, sweat inducers, and/or suction devices with or without the application of electrical current. One device recently approved by the FDA relies on reverse iontophoresis, utilizing an electrical current applied to the skin. The current pulls out salt, which carries water, which in turn carries glucose. The glucose concentration of this extracted fluid is measured and is proportional to that of blood. This technology, in keeping with its nearly noninvasive description, is commonly associated with some discomfort and requires at least twice daily calibrations against conventional blood glucose measurements (e.g., invasive lancing).
Other nearly noninvasive blood glucose monitoring techniques similarly involve transcutaneous harvesting for interstitial fluid measurement. Other technologies for disrupting the skin barrier to obtain interstitial fluid include: 1) dissolution with chemicals; 2) microporation with a laser, sound, or electrical stimulation; 3) penetration with a thin needle; and/or 4) suction with a pump. Minimally invasive blood glucose monitoring can also involve the insertion of an indwelling glucose monitor under the skin to measure the interstitial fluid glucose concentration. These monitors typically rely on optical or enzymatic sensors. Technologically innovative, these in situ sensors have had limited success. Implantable glucose oxidase sensors have been limited by local factors causing unstable signal output, whereas optical sensors must overcome signal obfuscation by blood constituents as well as interference by substances with absorption spectra similar to glucose. Moreover, inflammation associated with subcutaneous monitoring may contribute to systematic errors requiring repositioning, recalibration or replacement, and more research is needed to evaluate the effects of variable local inflammation at the sensor implantation site on glucose concentration and transit time.
Interstitial fluid glucose concentrations have previously been shown to be similar to simultaneously measured fixed or fluctuating blood glucose concentrations (Bantle et al., Journal of Laboratory and Clinical Medicine 130:436-441, 1997; Sternberg et al., Diabetes Care 18:1266-1269, 1995). Such studies helped validate noninvasive/minimally invasive technologies for blood glucose monitoring, insofar as many of these technologies measure glucose in blood as well as interstitial fluid.
A noninvasive glucose monitor that is portable, simple and rapid to use, and that provides accurate clinical information is highly desirable. In particular, the ability to derive primary and secondary order information regarding real time, dynamic glucose metabolism (such as the direction and rate of change of bioavailable glucose distributed within the blood and interstitial fluid space) is highly desirable.